DWR

DWR is AJAX and XMLHttpRequest made easy. It makes it simple to call Java on the server directly from Javascript in the browser marshalling all the parameters and return values.
It gets rid of almost all the boiler plate code between the web browser and your Java code.
interaction diagram
DWR helps you in producing highly interactive web-sites by providing some Javascript libraries to help with DHTML and by giving you a set of examples to copy from.